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We present a new multiparty controlled quantum secure direct communication protocol with phase encryption, in which the sender's secret message can only be recovered by the receiver under the permission of all the controllers. The security of this scheme is based on the basic principles of quantum mechanics and the secret order of encoding photons.

The research on decoherence-free subspace has both theoretical and practical significance. DFS is a subspace of quantum system states which is robust against a specific form of decoherence. The method of constructing DFS based algebra, which is different from the way based on the special Bell states, is introduced. Then a three-particle QKD protocol against collective-bit flipping noise is presented according to this method. Finally, we make a security analysis of our protocol.

Perfect teleportation and superdense coding are discussed via a special kind of W-state. It is shown that the state can be used for perfect teleportation of the state x|0〉⊗N + y|1〉⊗N. And the state can be utilized for superdense coding. Moreover, it is demonstrated that the sender can transmit N classical bits to the receiver by sending N − 1 qubits.

A quantum key distribution (QKD) protocol based on constructed nonorthogonal states is proposed. By transmitting the photons from Bell states in random order the sender constructs nonorthogonal states, which can prevent an eavesdropper from extracting the key information. At the destination, the receiver performs Bell measurements and then obtains secure key. Different from previous nonorthogonal-states-based QKD protocols, our protocol exhibits a new feature, that is, the users can also obtain some key bits even when they choose different "bases"! Thus it can reach a higher efficiency in theory. Besides, we present a trade-off between the information that an eavesdropper can elicit and the corresponding error rate he/she would introduce, which shows that our protocol is secure.

A two-step channel-encrypting quantum key distribution protocol is proposed. Using the previously shared EPR pairs as the quantum key, two bits of classical key can be established via one information carrier EPR state on average. In theory, the efficiency of this protocol reaches 100%, and there is no need to consume any entangled states including both the quantum key and the information carriers in ideal condition. The protocol can resist the particular attack that is fatal to other some channel-encrypting schemes. Principally, we prove the security against the most general individual attack of this protocol. Entanglement collapse in practical situation, as well as the realistic implementation of this protocol is also discussed.

We analyze the security of multiparty controlled quantum secure direct communication using Greenberger–Horne–Zeilinger (GHZ) state. It is shown that the receiver, using a special property of GHZ state, can illegally obtain 33.3% of the sender’s secret without any controller’s permission. The attack strategy is demonstrated in detail and an improvement of this protocol is discussed. The idea of this attack might be instructive for the cryptanalysis of quantum cryptographic protocols.

Firstly, we investigate the necessary and sufficient conditions that an entangled channel of n -qubits should satisfy to carry out perfect teleportation of an arbitrary single qubit state and dense coding. It is shown that the sender can transmit two classical bits of information by sending one qubit. Further, the case of high-dimension quantum state is also considered. Utilizing n -qudit state as quantum channel, it is proposed that the necessary and sufficient conditions are ( d + 2 ) ( d - 1 ) 2 in all to teleport an arbitrary single qudit state. The sender can transmit 2 log 2 d classical bits of information to the receiver conditioned on the constraints.

A scalable protocol for multiparty quantum secret splitting with collective eavesdropping-check is proposed by using Einstein–Podolsky–Rosen pairs. We analyze the security of this protocol and prove that it can stand against some possible attacks in an ideal condition. Meanwhile, this protocol utilizes quantum dense coding to achieve a high intrinsic efficiency and source capacity. Moreover, only Bell-state measurement and local unitary operations are required, which makes this protocol more convenient from an applied point of view.

A multiparty quantum secret sharing scheme based on Bell measurement is proposed and analyzed. In this scheme, all agents are not required to prepare entangled states or perform any local unitary operation. The security of the protocol is also analyzed. It is shown that any eavesdropper will introduce errors invariably and be detected if he tries to steal information about Trent’s secret. Moreover, because no classical bit needs to be transmitted except those for detection, the total efficiency of the scheme approaches to 100%.

We present an efficient scheme for secure authentication of classical messages with decoherence-free states and hash functions. We also analyze the security of this scheme and show it is not possible to forge valid message authentication codes. In addition, the lengths of the authentication key and message authentication codes are invariant no mater how long the length of the message to be authenticated is. Moreover, although this scheme uses entangled states for encoding, only single-particle product measurements are required.

An efficient quantum secure direct communication protocol is presented over the amplitude damping channel. The protocol encodes logical bits in two-qubit noiseless states, and so it can function over a quantum channel subjected to collective amplitude damping. The feature of this protocol is that the sender encodes the secret directly on the quantum states, the receiver decodes the secret by performing determinate measurements, and there is no basis mismatch. The transmission’s safety is ensured by the nonorthogonality of the noiseless states traveling forward and backward on the quantum channel. Moreover, we construct the efficient quantum circuits to implement channel encoding and information encoding by means of primitive operations in quantum computation.

A scheme of multiparty quantum secret sharing of classical messages (QSSCM) is proposed based on single photons and local unitary operations. In this scheme, eavesdropping checks are performed only twice, and one photon can generate one bit of classical secret message except those chosen for eavesdropping check; in addition, only the sender and one of the agents are required to store photons. Thus, this scheme is more practical and efficient.

A quantum secret sharing (QSS) protocol between multiparty and multiparty is proposed, based on Greenberger–Horne– Zeilinger (GHZ) state. The protocol utilizes quantum Fourier transform and entanglement swapping to achieve a high intrinsic efficiency and source capacity. Then, the security of this protocol against some possible eavesdropping strategies has been analyzed. Furthermore, the presented protocol is generalized to the d-level case.

A scheme of multiparty quantum secret sharing of classical messages (QSSCM) [Z.J. Zhang et al., Opt. Commun. 269 (2007) 418] was proposed. Lin et al. [S. Lin et al., Opt. Commun. 281 (2008) 4553] showed the last agent can obtain half of the secret in Z.J. Zhang's et al. three-party QSSCM scheme and gave an improved version. We further show the first agent and the last agent can obtain all the secret without introducing any error in Zhang's et al. multiparty QSSCM scheme by a special attack with quantum teleportation. We also present an improved version.

The way to compare the efficiencies of different detect strategies (DSs) in the “ping-pong” protocol is studied. The trade-off between information gain and disturbance is calculated and compared for different DSs. The comparison result primely tallies with our intuitional analysis. It is shown that the analysis of this trade-off is a feasible way to compare the performances of different DSs in theory.

Utilizing W state, which is much more robust than GHZ state, we propose a protocol for three-party controlled quantum secure direct communication (QSDC). The feature of this protocol is that the sender encodes the secret message directly on a sequence of particle states and faithfully transmits them to an arbitrary one of two receivers without revealing any information to a potential eavesdropper. On the other hand, we construct the efficient quantum circuit to implement the QSDC by means of primitive operations in quantum computation.

Utilizing a partially entangled GHZ state, we propose a novel controlled quantum secure direct communication (QSDC) with quantum encryption. Under the supervision and help of the third side, the sender and the receiver can securely share the private quantum entanglement keys used to encrypt and decrypt the secret message. According to the results of checking the eavesdropping on decoy photons, communicators can decide whether the quantum keys are reused in the next round. Not only will eavesdropping inevitably disturb the states of the decoy photons and be detected, but arbitrary transmission errors can also be corrected.

The GHZ-state-based quantum secure direct communication (QSDC) protocol [X.-R. Jin, et al., Phys. Lett. A 354 (2006) 67] and its improved version [Z. Man, Y. Xia, Chin. Phys. Lett. 24 (2007) 15] are analyzed from the aspect of security. It shows that much information of the transmitted secret message will be leaked out in both protocols.

A protocol for three-party controlled quantum teleportation is discussed. It is shown that an unknown two-particle entanglement can be teleported to any one of two receivers via the only one three-particle W state. We summarize all different unitary transformations performed by the receiver with a concise formula. The quantum circuits for the generalized measurement described by positive operator-valued measure (POVM), which is utilized to probabilistically distinguish the two non-orthogonal states, are explicitly constructed by means of quantum Toffoli gates. The efficient quantum circuits for implementing the teleportation are also provided.

In this article, a protocol for the teleportation of an unknown two-particle entanglement is proposed. The feature of the present protocol is that we utilize an asymmetric three-particle entangled state as the quantum channel. The optimal discrimination between two nonorthogonal quantum states is adopted. It is shown that an unknown two-particle entangled state can be probabilistically teleported from the sender to the remote receiver on condition that the co-sender successfully collaborates. The fidelity in this protocol is one. In addition, the probability of the successful teleportation is calculated and all kinds of transformations performed by the sender and the receiver are provided in detail.

The two quantum secret sharing protocols in the paper of Bradler and Dusek (2004 J. Opt. B: Quantum Semiclass. Opt. 6 63) are analysed and it is shown that both protocols are insecure not only for a dishonest participant but also for an external eavesdropper. An external eavesdropper Eve can steal half of Alice's secret without being detected, if she employs a special attack strategy. We give a possible way to improve the protocol to withstand this attack.

It deserves mentioning that the quantum circuit, i.e. quantum logic network, is essential to the practical realization of teleportation in experiment. Using only one partially entangled pair, we first propose two novel strategies for probabilistically teleporting any partially entangled state of a bipartite system. The feature of the present protocol is to weaken the requirement for the quantum channel, and also to cut down the number of entangled particles initially shared by the sender and receiver. On the other hand, we explicitly construct the generalized measurement described by the positive operator-valued measure (POVM). Two kinds of efficient quantum circuits for implementing the teleportation are offered. In addition, we generalize the two-particle probabilistic teleportation to the system of many particles.

The ring-arrangement quantum secret sharing protocol in the paper [K. Br\'{a}dler and M. Du\v{s}ek (2004) {\textit{J. Opt. B: Quantum Semiclass. Opt.}} {\textbf{6}} 63] is analyzed and it is shown that this protocol is secure for any eavesdropper except for a dishonest participant. For example, by a special strategy, Bob can steal Charlie's portion of information without being detected and then recover Alice's secret by himself. We give a description of this strategy and point out a possible way to improve the protocol to stand against this attack.

A scheme for teleporting an unknown two-particle entangled state is proposed. In comparison with the recent protocol (Cola et al., Phys. Lett. A 337 (2005)), the entangled state as quantum channel required by this scheme is a single, partially entangled pair, which is much easier to prepare and maintain. Furthermore, a positive operator valued measure (POVM) is adopted and all kinds of transformations performed by sender and receiver are given in detail. It is shown that the probability of successful teleportation is twice the modulus square of the smaller Schmidt coefficient of the two-particle entangled state, and the fidelity can reach one.

Based on entanglement swapping between two Bell states, two novel quantum key distribution protocols are proposed. One is for two-level systems, where there is no need for classical communication before each entanglement swapping. This feature is essential to its practical realization. Furthermore, to establish an arbitrarily long key, the protocol needs only two Bell states. The other is for d-level (d > 2) systems, in which higher security and higher source capacity are achieved. Using the theory of quadratic residue, we prove that in the two-qudit systems, each Bell state is a uniform superposition of all basis states in the dual basis, which is different to the situation in two-qubit systems. This difference means our two-level protocol cannot be generalized to the d-level situation directly. On the other hand, it results in higher security of our d-level protocol and is instructive to design quantum cryptography protocols.

With the help of a simple quantum key distribution (QKD) scheme, we discuss the relation between BB84-type protocols and two-step-type ones. It is shown that they have the same essence, i.e., information-splitting. More specifically, the similarity between them includes (1) the carrier state is split into two parts which will be sent one by one; (2) the possible states of each quantum part are indistinguishable; (3) anyone who obtains both parts can recover the initial carrier state and then distinguish it from several possible states. This result is useful for related scheme designing and security analyzing., Comment: 4 pages, 1 figure, Revtex 4

The security of multiparty quantum secret sharing with Bell states and Bell measurements [R.H. Shi et al., Opt. Commun. 283 (2010) 2476] is analyzed. It is shown that the first agent and the last one can gain access to the dealer's secret if they collaborate in this protocol. Therefore, this protocol does not satisfy the security requirement of quantum secret sharing.

The security of quantum secure direct communication by entangled qutrits and entanglement swapping [Y.B. Zhan et al., Opt. Commun. 282 (2009) 4633] is analyzed. It is shown that an eavesdropper can obtain all the secret without being found by a simple intercept-and-resend attack. Finally, a possible improvement to resist this attack is proposed.

The security of the multiparty quantum secret sharing protocol presented by Zhang [Z.J. Zhang, Physica A, 361 (2006) 233] is analyzed. It is shown that this protocol is vulnerable to the insider attack since eavesdropping detection is performed only when all states arrive at the last agent. We propose an attack strategy and give an improved version of the original protocol. The improved protocol is robust and has the same traits with the original one.

The security of a secure quantum sealed-bid auction protocol using quantum secure direct communication [Mosayeb Naseri, Opt. Commun. 282 (2009) 1939] is analyzed. It is shown that this protocol is unfair as a malicious bidder can obtain others’ bids without being found, and then he can optimize his bid to win the auction. Finally, a simple improvement to resist this attack is proposed.

The security of a deterministic secure quantum communication using four-particle genuine entangled state and entanglement swapping [X.M. Xiu, H.K. Dong, L. Dong, Y.J. Cao, F. Chi, Opt. Commun. 282 (2009) 2457] is analyzed. It is shown that an eavesdropper can entangle an ancilla without introducing any error in the security test utilizing a speciality of the four-particle genuine entangled state. Moreover, the eavesdropper can distill a quarter of the secret information from her entangled ancilla. Finally, a simple improvement to resist this attack is proposed.

A controlled deterministic secure quantum communication protocol [X.M. Xiu, L. Dong, Y.J. Cao, F. Chi, Opt. Commun. 282 (2009) 333] with five-qubit entangled states was proposed recently. The aim of Xiu et al. was that the successful realization of communication between Alice and Bob needed the cooperation of a controller, Charlie. However, we show that the controller Charlie’s role could be excluded unknowingly. Moreover, an eavesdropper can entangle an ancilla without introducing any error in the first security test and then she can distill a quarter of the secret messages from her entangled ancilla.

The security of the secure quantum telephone protocol [X.J. Wen, Y. Liu, N.R. Zhou, Opt. Commun. 275 (2007) 278] is analyzed. It is shown that an eavesdropper can attack the communicators’ messages by using fake particles and local operations. Moreover, the essential reasons of the information leakage are discussed. Finally, a simple improvement of the secure quantum telephone protocol is proposed.

The security of a multiparty quantum secret sharing protocol [L.F. Han, Y.M. Liu, J. Liu, Z.J. Zhang, Opt. Commun. 281 (2008) 2690] is reexamined. It is shown that any one dishonest participant can obtain all the transmitted secret bits by a special attack, where the controlled- ( - i σ y ) gate is employed to invalidate the role of the random phase shift operation. Furthermore, a possible way to resist this attack is discussed.

The security of quantum exam [B.A. Nguyen, Phys. Lett. A 350 (2006) 174] is analyzed and it is found that this protocol is secure for any eavesdropper except for a dishonest “student” who takes part in the exam. The particular attack strategy is demonstrated and a possible improvement of this protocol is presented.

We come up with a special attack strategy to the multiparty quantum secret sharing protocol [Z.J. Zhang, Y. Li, Z.X. Man, Phys. Rev. A 71 (2005) 044301]. Using fake signal and Bell measurement, the agent Bob who generates the initial signals can elicit Alice's secret message. With little modification, our strategy also works for an improved version of this protocol. At last, a further improvement of the multiparty quantum secret sharing protocol is proposed.

The security of the decoherence-free version of the Bennett-Brassard 1984 (BB84) protocol [A. Cabello, Phys. Rev. A 75, 020301 (2007)] is analyzed and shown to be vulnerable under the intercept-resend attack. We propose two improved versions of this protocol. Both improvements remain the performance of robustness against collective noise and refuse the security flaw. Especially, the second improvement, which is called four-qubit decoherence-free (DF) BB84 protocol, not only remains all characteristics of the original protocol but also has a higher efficiency. We also give a detailed security proof of four-qubit DF BB84 protocol.

A collective photon-number-splitting attack strategy is proposed, which combines photon-number-splitting attack with an unambiguous set discrimination of quantum state. Verified by this attack strategy, it is shown that a two-way quantum secure direct communication protocol with qubits is insecure in real circumstance. Finally, we present a possible improved version of this kind of quantum secure direct communication protocol.

In this paper, a quantum secure direct communication protocol with $\ensuremath{\chi}$-type entangled states ${\ensuremath{\mid}{\ensuremath{\chi}}^{00}⟩}_{3214}$ is proposed. We analyze the security of this protocol and prove that it is secure in ideal conditions. Then, an alternative way is presented to ensure the security of this protocol in a noisy channel. Moreover, this protocol utilizes quantum superdense coding to achieve a high intrinsic efficiency and source capacity. The practical implementation of this protocol is also discussed.

A Comment on the Letter by [Sascha Gaertner et al., Phys. Rev. Lett. 100, 070504 (2008)]. The authors of the Letter offer a Reply.

The participant attack is the most serious threat for quantum secret-sharing protocols. We present a method to analyze the security of quantum secret-sharing protocols against this kind of attack taking the scheme of Hillery, Buzek, and Berthiaume (HBB) [Phys. Rev. A 59 1829 (1999)] as an example. By distinguishing between two mixed states, we derive the necessary and sufficient conditions under which a dishonest participant can attain all the information without introducing any error, which shows that the HBB protocol is insecure against dishonest participants. It is easy to verify that the attack scheme of Karlsson, Koashi, and Imoto [Phys. Rev. A 59, 162 (1999)] is a special example of our results. To demonstrate our results further, we construct an explicit attack scheme according to the necessary and sufficient conditions. Our work completes the security analysis of the HBB protocol, and the method presented may be useful for the analysis of other similar protocols., Revtex, 7 pages, 3 figures; Introduction modified

In a recent Letter [G. Chiribella et al., Phys. Rev. Lett. 98, 120501 (2007)], four protocols were proposed to secretly transmit a reference frame. Here We point out that in these protocols an eavesdropper can change the transmitted reference frame without being detected, which means the consistency of the shared reference frames should be reexamined. The way to check the above consistency is discussed. It is shown that this problem is quite different from that in previous protocols of quantum cryptography., 3 pages, 1 figure, comments are welcome

Based on quantum encryption, we present a new idea for quantum public-key cryptography (QPKC) and construct a whole theoretical framework of a QPKC system. We show that the quantum-mechanical nature renders it feasible and reasonable to use symmetric keys in such a scheme, which is quite different from that in conventional public-key cryptography. The security of our scheme is analyzed and some features are discussed. Furthermore, the state-estimation attack to a prior QPKC scheme is demonstrated., Comment: 8 pages, 1 figure, Revtex4

From the perspective of information theory and cryptography, we analyze the security of two quantum dialogue protocols and a bidirectional quantum secure direct communication (QSDC) protocol, and point out that the transmitted information would be partly leaked out in them. That is, any eavesdropper can elicit some information about the secrets from the public annunciations of the legal users. This phenomenon should have been strictly forbidden in a quantum secure communication. In fact, this problem exists in quite a few recent proposals and, therefore, it deserves more research attention in the following related study., Comment: An authors' note was added. That is, when this study was completed, we found that the insecurity of quantum dialogue was independently pointed out by Tan and Cai (see arXiv:0802.0358)